It is common medical practice to directly measure blood pressure of critically ill patients. In contrast to the intermittent measurement of systolic and diastolic blood pressures using an inflated cuff on the arm and a stethoscope, direct pressure measurement allows continuous unattended monitoring of the patient's status.
In brief, direct blood pressure measurement is done as follows: a small tube called a catheter is placed in an artery. Typically, this is the radial artery in the wrist. A length of extension tubing connects the catheter to a pressure transducer. The transducer dome, the extension tubing, and the catheter are filled with saline solution. Various valves, stopcocks, and flush devices allow for filling the tubing system with fluid, for flushing the catheter with fluid to prevent clotting of the catheter tip, and for the withdrawal of blood samples for diagnostic tests.
The pressure signal detected by the pressure transducer is amplified and displayed on a monitoring oscilloscope. This allows direct viewing of the pressure wave generated by the beating of the patient's heart. The amplified pressure signal is also sent to circuitry which extracts the highest and lowest pressures found in the wave. These are known, respectively, as the systolic and diastolic pressures, and are typically displayed on the blood pressure monitor. Therapy is guided by the measured systolic and diastolic pressures.
In order for the blood pressure monitor to function properly, the pressure measured at the transducer must be an accurate replica of the pressure in the patient's blood vessel at the tip of the catheter. If the blood pressure were constant, having no pulsatile component, only the difference in height, and the resulting hydrostatic head between the heart and the transducer would have to be taken into account. However, this is not the case and so with the pressure pulses found in both the systemic and pulmonary arteries, the dynamic properties of the fluid filled tubing and the associated valves and transducer must be carefully controlled to obtain accurate transmission of the pressure waves from the blood vessel to the pressure transducer.
There are two common situations which lead to clinically significant distortion of the pressure wave, and to incorrect systolic and diastolic values being measured. The first is the use of inappropriate tubing and/or valves. If the tubing is too long, too small in diameter, or too soft and compliant, wave form distortion results. The second is the presence of small air bubbles in the fluid filled system. The presence of bubbles, especially near or at the transducer, produces resonant behavior. For example, a bubble of 3 cubic millimeters volume near the transducer in a typical clinical setup may cause the systolic pressure to read 10 to 30 mmHg. higher at the transducer than in the catheterized artery. In this case, the fluid in the tubing and the bubble act in analogy to a mechanical mass and spring resonant system. Potential energy stored as compression of the bubble is exchanged with kinetic energy of the fluid moving in the tubing. In a similar way, if the tubing itself or the transducer has a significant change in volume associated with a change in pressure, the system is said to have an excessively high compliance. The potential energy associated with compliance of tubing or transducer causes resonant behavior of the system by the same mechanism as the potential energy stored in an air bubble.
In order to calibrate such direct pressure measurement devices and to instruct personnel in their proper care and operation, blood pressure wave simulators are used. Such simulators commonly employ a pressure generator including an electromagnetic coil. The coil suspension is stiff so that the compliance of the generator is within an order of magnitude of the tubing compliance. Thus the pressure in the generator is responsive to the fluid displacement in the tubing and the generated pressure wave varies as a function of the compliance of that tubing. These stiff suspension simulators are often modified by placing a large air bubble in the otherwise non-compliant fluid chamber at the diaphragm. In one case the bubble is 10 cc; while the introduction of an air bubble tends toward solving the compliance problem, it introduces two other problems. First, substantial displacement of the stiff coil suspension is required to compress the bubble sufficiently to develop the required pressure, and because the spring constant is high the net force transmitted to the fluid is small compared to the force generated by the current in the coil; and second, because the spring force is high compared with the force transmitted to the fluid, the developed force is very dependent on the volume of the compliant air bubble in the chamber. Therefore the bubble's size must be controlled, which introduces another step in the procedure of setting up the simulator.